Composite members created by welding an outer peripheral resin member to the peripheral surface of a core metal have conventionally been used for various parts and devices. When using such a composite member to a worm wheel of a power steering device, it is required to bond the core metal and the outer peripheral resin member securely to ensure secure steering and good durability of the device.
Some techniques for welding an outer peripheral resin member to the periphery of a core metal by fitting the core metal into a fitting hole of the outer peripheral resin member and then subjecting them to induction heating have been proposed. For example, the following Patent Literature 1 discloses a method of fastening a metal boss to a resin formation. With this method, forming concave-convex shape on the surface contacting the resin, namely the peripheral surface of the metal boss, is conducted first. On the resin formation, a through hole having a diameter slightly smaller than the outer diameter of the metal boss is formed. A bonding material is applied to the roughened surface of the metal boss, the resin formation is heated to expand the diameter of the through hole, and the metal boss is pressure-fed into the through hole. Then, by conducting high-frequency induction heating, the part of the resin formation contacting the metal boss is heated to the melting temperature or higher to weld the resin formation to the metal boss. According to the method of cited literature 1, molded articles such as flat gears and wheels made of a resin and metal can be manufactured efficiently.
Patent Literature 2 discloses a method of manufacturing a worm wheel. According to this method, outer peripheral concaves and convexes lined up in the circumferential direction are formed first on the outer periphery of a metal core metal, and on the inner periphery of a resin ring gear, inner peripheral concaves and convexes lined up in the circumferential direction are formed in a shape matching the outer peripheral concaves and convexes of the core metal. The outer peripheral concave-convex portion and the inner peripheral concave-convex portion are fitted with an adhesive agent sandwiched between the ring gear and the core metal, and high-frequency welding is then performed for bonding. Cited literature 2 asserts that the degree of freedom in design can be improved while the fixing strength between the metal core metal and the resin ring gear is maintained.
With the conventional technique of performing induction heating with a core metal fitted into a fitting hole on the outer peripheral resin member, as disclosed both in Patent Literatures 1 and 2, concaves and convexes are formed on the peripheral surface of the core metal to improve the bonding strength by welding between the outer peripheral resin member and the core metal. For example, in Patent Literature 1, knurling of 1 to 3 mm is conducted to improve bonding strength. The cited literature 2 forms male serrations on the outer periphery of the metal core metal to improve bonding strength.